In the fields of integrated optical devices and electro-optical devices, polarization mode converters are known. These converters receive light polarized in a first mode and provide light at an output in a second polarization mode. Mode converters can be either active or passive. An example of an active mode converter is disclosed in U.S. Pat. No. 5,566,257 to Jaeger et al., which is incorporated herein by reference. As used herein, “mode converter” refers to a polarization mode converter.
The basic function of an electro-optic mode converter can be described by extending the concept of principal optical axes for plane waves in birefringent media to optical waveguides. Light is confined in the optical waveguide in a small cross-sectional area, transverse to the direction of propagation. The mode converter is constructed to allow only the two fundamental optical modes to propagate. These modes are nearly identical in their optical intensity distribution, but differ in their polarization. FIG. 1 illustrates a conventional semiconductor optical mode converter 50. The electro-optical mode converter 50 typically includes an optical guiding layer 52, such as a III-V semiconductor or LiNbO3-type material, formed on an electro-optical substrate 52. An optical waveguide 54 is defined on, or in, the substrate, and an electrode structure 58a, 58b is disposed in the vicinity of the optical waveguide.
The mode converter 50 is constructed to support two fundamental optical modes, one polarized predominantly along the U axis and the other predominantly polarized along the V axis. Light launched into either of these modes will propagate through the mode converter and maintain the optical power (excluding optical power loss) and polarization state. In an ideal mode converter, the principal axes, U and V, are oriented 45° from the chip axes, X and Y. Light launched into the waveguide polarized in either of the X or Y directions is resolved vectorially into equal amounts of U and V polarized components and propagates along the U and V axes of the waveguide. Light polarized in the X and Y directions is referred to as TE polarized and TM polarized, as is the convention with planar optical waveguides. Due to a phenomenon known as birefringence, where each of the principal axes has a different index of refraction, light in the U and V axes will travel at different phase velocities, leading to a “fast axis” and a “slow axis”. Speed along either axis can be faster than the other depending on the construction of the mode converter. The degree of difference between the phase velocities of the fast and slow axes is called the modal birefringence of the mode converter. At the output of the waveguide, the light from each of the principal axes combines to form the output light.
In an ideal mode converter, equal powers are launched into the fast and slow axes of the waveguide. With the U and V axes oriented as in FIG. 1, TM polarized light 62, will resolve itself into equal powers in the fast and slow axes, as shown by arrows 62u and 62v in FIG. 2a. The polarization state at the output of the waveguide will be determined by the difference in the phase velocities of the fast and slow axes and the total length of the waveguide. An electric field can be applied to alter the polarization state, in other words, to obtain polarization modulation, by changing the difference in phase velocities. As a voltage is applied to the electrodes 58a, 58b, an electric field is formed across the optical waveguide, which can modify the optical properties of the waveguide material via the electro-optic effect, resulting in modified properties of the optical waveguide. The modified waveguide properties include the orientation of its principal axes and/or its birefringence, thus modifying the state of polarization of a light beam traversing the optical waveguide. In an ideal mode converter, the principal axes are rotated to 45° from an X-Y orientation, and the birefringence of the axes is adjusted so that the axes are either in phase or 180° out of phase with each other. This action of changing the phase velocities is known as switching the mode converter.
For example, when the difference in phase velocities is tuned to produce a 180° phase shift between the fast and slow modes at the waveguide output, light 62 launched with a linear polarization in the Y direction, as shown in FIG. 2a, will exit the waveguide as light 64 polarized in the X direction, as shown in FIG. 2b, where the constituent elements in the U and V axes are shown as arrows 62u′ and 62v′. This action can be described as a power conversion from a TE polarization to TM polarization, or polarization modulation. In FIG. 2c, the converter has been tuned to produce a 0° phase shift. Thus, the light 64 at the output is polarized in the Y direction. As FIGS. 2a-2c show, by tuning the mode converter, two orthogonal output polarization states can be created. For linear polarizations, two states are orthogonal if their orientations are separated by 90°.
Unfortunately, actual mode converters do not behave in the ideal manner described above. Due to manufacturing tolerances, unstable material stresses and other non-idealities, a real mode converter will typically depart from the ideal behaviour: the orientation of the principal axes, U and V, may not be exactly at 45° to the chip axes, X and Y; the optical loss along each of the U and V axes may differ; the orientation of the U and V axes may vary along the length of the chip; the optical loss in each of the U and V axes may vary along the length of the chip; and, the orientation of the U and V axes may change with the electric field applied. Any manufactured mode converter will exhibit some combination of these non-ideal behaviours or others to some degree, depending on the design and manufacturing process variations. If a non-ideal mode converter is configured in the same way as described for the ideal mode converter, with an input linear polarization aligned with the X or Y chip axes, then as the mode converter is modulated, the output polarization will not sweep through orthogonal states. Orthogonal states are particularly desirable when using the mode converter as a polarization modulator or intensity modulator.
It is, therefore desirable to provide an electro-optical mode converter that is optimized to provide orthogonal outputs, preferably having maximized power.